Optical cavities only interact with certain narrow, regularly spaced wavelengths of light called optical resonances. The ability to control (‘tune’) the wavelength of these resonances is important for a wide variety of applications. For example, a laser beam can only couple to a cavity if its wavelength precisely matches one of the resonances, and two distinct optical cavities cannot interact with each other unless they share a common resonance wavelength. Important capabilities include the ability to tune fast, with high efficiency, and over a wide frequency range.
For many optical cavities, and in particular for miniaturized cavities, the main currently used method to tune the resonance frequencies relies on thermal heating, whereby heating the resonator changes its size and refractive index and therefore shifts the optical resonances. However, thermal tuning is relatively slow, has low efficiency and, depending on the material, can be only operable over a narrow range due to damage to the optical cavity caused by very high or very low temperatures.
Tunability is important for a particular class of optical cavities termed ring cavities. In some known prior art devices the optical field may be confined in whispering gallery modes (WGM) within a micron-scale disk fabricated on a chip. In our earlier published international patent application WO2015/039171 we describe a related device that uses an optical microcavity as the basis for an optical magnetometer. Various optical cavities are described in WO2015/039171, the content of which is incorporated herein by reference.
The range of applications for tunable optical micro-cavities is quite broad, including biological, nanoparticle, and magnetic field sensing; on-chip precision clocks for GPS and other timing applications; and on-chip lasers, optical interconnects, and integrated optical delay lines for optical components in computation and communication. All of these applications are currently constrained by the lack of a fast, easy, efficient, and scalable way to tune the optical resonators; and typically rely instead on the tuneability of the laser sources themselves, which precludes the use of arrays of microresonators, and requires large and expensive laser sources which are not compatible with fully integrated solutions. Furthermore the emerging field of on-chip photonic circuitry will benefit from optical tuneability as well if networks of resonators are to be linked.